Support setting fan speeds

[Arnavion]

Needed to supersede https://github.com/Arnavion/tpfancontrol-rs with this.

Options:

1. Implement fan curve in sensord; no dynamic configuration

   Easy to implement, but somewhat redundant since fancontrol can already do this (albeit with a much simpler curve).

   TODO: What about driver-specific things like thinkpad_acpi's fan_watchdog ?

2. Implement fan curve in hwtop; no dynamic configuration

   Needs sensord to expose a dbus method for fn set_fan_speed(hwmon: &str, num: u8, pwm: Pwm), where enum Pwm { Auto, FullSpeed, Manual(u8), which sets pwm{}_enable and pwm{} accordingly. The method should be ACLed so that arbitrary users can't call it.

   TODO: What about driver-specific things like thinkpad_acpi's fan_watchdog ?

3. Implement fan curve in hwtop; dynamic configuration

   Same as (2), but also needs UI to let the user choose the fan mode like tpfancontrol-rs has. Complex without bringing in the deps that I moved away from in the first place.